1. Field of the Invention
In general, the present invention relates to an exposure apparatus for manufacturing semiconductor devices, liquid crystal substrates, or imaging devices using lithography. In particular, the present invention relates to an exposure apparatus suitable for transferring a pattern formed on an original plate onto a photosensitive substrate through a projection optical system by performing suitable irradiation of the original pattern using a laser for generating pulsed light in the ultraviolet range, or to a device for emitting light in the extreme ultraviolet range as a light source.
2. Description of the Related Art
Within the semiconductor device manufacturing process, photolithography is performed, wherein irradiation light is cast onto an original plate having a desired circuit pattern formed on the surface thereof, which is referred to as “photomask” or “reticle” (hereinafter referred to as “mask”). This results in transferring of the pattern formed on the mask onto a photosensitive substrate (hereinafter referred to as “wafer”) formed of a silicon or glass substrate of which the surface has been coated with a photosensitive material. An exposure technique for transferring the pattern with an equal magnification or a reduced magnification is used.
Using λ as the wavelength of the exposure light employed for projection exposure, and NA as the numerical aperture of the projection optical system, in general, the resolution R is represented by the following expression, where k1 represents a coefficient:R=k1·λ/NA  (Expression 1)
Accordingly, improvement of the resolution R has been undertaken by increasing the numerical aperture NA and reduction of the wavelength of the irradiation light. As can be understood from the aforementioned expression, with the same numerical aperture, the resolution is improved as the wavelength of the irradiation light become shorter. That is, the resolution of the projection optical system is improved by replacing an extra-high pressure mercury lamp serving as a light source for projection for emitting an emission line such as the g line or the i line, with a KrF excimer laser with the wavelength of 248 nm. Furthermore, the light source for projection may be replaced by an ArF excimer laser with the wavelength of 193 nm, a F2 excimer laser with the wavelength of 157 nm, or an EUV light source with the wavelength of 4 to 20 nm, the resolution being further improved in that order.
On the other hand, with such an exposure apparatus employing an ArF excimer laser as a light source, for example, the range of the spectrum lines of the exposure light is included in the absorption spectrum range of oxygen, which leads to reduction of light efficiency due to oxygen absorbing the light. In addition, ozone occurs due to the absorption, further leading to reduction of the transmissivity of the optical system.
Even with an arrangement having a configuration wherein the wavelength bandwidth of the ArF laser beam is narrowed down so as to suppress absorption of light due to oxygen, the improved transmissivity is only approximately 98% for each optical path of 1 m.
Furthermore, the occurrence of ozone leads to chemical reaction of impurities such as moisture, hydrocarbons, or organic matter in the optical path, and the deposited impurities adhere to the surfaces of the optical devices, leading to reduction of the transmissivity of the optical system.
As an arrangement for solving the above-described problems such as reduction of the transmissivity of the optical system and the like, an exposure apparatus is disclosed in Japanese Patent Laid-Open No. 2000-091207, having a configuration wherein the inside of the optical system is filled with an inert gas such as nitrogen, helium, or the like, so as to eliminate impurities from forming reaction deposit on the optical path. Furthermore, the aforementioned exposure apparatus has an improved configuration wherein even in the event that impurity deposition occurs on the surfaces of the optical devices, the surfaces of the optical devices are cleaned with so-called photo-cleaning, thereby maintaining excellent transmissivity of the optical system.
The photo-cleaning employed in the aforementioned exposure apparatus works by employing the phenomenon wherein irradiation of the face to be cleaned, using light with a wavelength between 100 nm and 200 nm, causes for example, separation of the impurities deposited on or adhering to the surface of the face into suspended particles in the air. That is, with the photo-cleaning, exposure light such as an ArF laser beam is cast onto the surface of an optical device of which the transmissivity has deteriorated, so as to cause separation of the impurities adhering to the surface of the face into suspended particles in the air. Then the purge gas such as an inert gas with which the inside of the optical system including the optical device has been filled, is replaced by a new one so as to eliminate the impurities from the optical path.
In general, optical devices employed along with an excimer laser such as a KrF laser or an ArF laser serving as a light source, are formed of quartz or fluorite. In particular, with optical devices formed of quartz, it is known that in the event that on-time and off-time of irradiation are alternately performed, the transmissivity of the material changes corresponding to the on-time and the off-time of irradiation.
FIG. 7 shows the transmissivity of an optical system including optical devices formed of quartz in the event that intermittent irradiation of the optical system has been repeated using an ArF laser, wherein the transmissivity 84 immediately following irradiation which has been performed again exhibits a greater value than the transmissivity 83 immediately before off-time 82, and the optical system exhibits rapid reduction of the transmissivity following irradiation which has been performed again. While the projection exposure apparatus has a configuration for adjusting the output of the laser serving as a light source while monitoring reduction of the transmissivity so as to correct exposure so as to obtain stable irradiation, it is difficult to correct the rapid change of the transmissivity occurring immediately following irradiation as described above. Accordingly, with the aforementioned projection exposure apparatus, tens of thousands of non-exposing laser pulses are emitted prior to exposure following off-time for stabilizing the transmissivity of the optical system (hereinafter referred to as “pre-exposure”), which has been employed as an effective method for suppressing rapid change of the transmissivity (see for example U.S. Pat. No. 6,163,365).
As described above, the aforementioned exposure apparatus needs to perform the processes for irradiating the irradiation optical system and the projection optical system with irradiation light, such as the aforementioned photo-cleaning, pre-exposure, and the like, in order to maintain the optical performance thereof, in addition to exposure of wafers. However, the above-described conventional light-source device has disadvantages as follows.
First, the irradiation light for photo-cleaning or pre-exposure emitted from the light source passes through the same optical path as the exposure light for exposure process. Accordingly, the irradiation light is focused at a position on the surface of the wafer. In the event that the irradiation light is cast onto the wafer, the exposure portion of the wafer is exposed, and the exposure light must be cast onto a portion other than the wafer.
However, as depicted in FIG. 1, a wafer stage 32 includes stage mirrors 35 and 36 for performing measurement of the position of the wafer stage 32 using a laser interferometer (not shown), an illumination sensor 33 for measuring exposure irradiation on the surface of a wafer 30.
Accordingly, the exposure light may be scattered due to reflection or the surface shapes of these devices, leading to the wafer 30 being exposed.
Next, there is a problem of thermal deformation. For example, FIG. 6 is a schematic plan diagram which shows distribution of the temperature on the wafer stage 32 in the event that the exposure light is cast onto the illumination sensor 33 mounted on the wafer stage 32. In the drawing, upon exposure of the illumination sensor 33, the temperature of the sensor increases due to the energy of the exposure light. The surface temperature of the stage mirrors 35 and 36, and the wafer stage 32 surrounding the mirrors, increases as a result of the increase in the temperature of the illumination sensor 33. The increase in temperatures leads to the deterioration of the precise driving and positioning of the wafer stage 32. FIG. 6 shows a case where exposure light is cast onto the illumination sensor 33. In a case of exposure for photo-cleaning, or pre-exposure, exposure is made at a predetermined position on the wafer stage 32 in the same way thus, deterioration of the precise driving and positioning of the wafer stage 32 due to heating occurs under these exposure scenarios as well.
Several methods are known for blocking the exposure light emitted for photo-cleaning or pre-exposure employed in conventional projection exposure apparatuses. For example, FIG. 11 shows a projection exposure apparatus having a configuration disclosed in Japanese Patent Laid-Open No. 10-335235 (corresponding to U.S. Pat. No. 6,268,904), wherein the irradiation light cast onto a mask 21 is cast onto the wafer 30 through a projection optical system 25. A shutter 51 for blocking the irradiation light is mounted such that it can be inserted between the projection optical system 25 and the wafer 30.
With an arrangement having the above-described configuration, the shutter 51 blocks the irradiation light cast onto the wafer 30 or the wafer stage 32 during the above-described photo-cleaning or pre-exposure. Use of the shutter 51 to block the irradiation light prevents an increase of the temperature on the surface of the wafer stage 32 that would otherwise occur due to the irradiation light being cast onto the wafer stage 32. However, as discussed above, with the projection exposure apparatuses of recent years, in general, the resolution R is represented by the aforementioned Expression (1). As can be understood from the Expression (1), the resolution is proportional to the wavelength of the irradiation light, and inversely proportional to the numerical aperture NA of the projection optical system 25. Accordingly, the projection optical system 25 preferably employs the irradiation light with a small wavelength, and preferably has a large aperture number NA. However, with a large aperture number NA, the optical system 25 must have a configuration with a small distance between the final lens thereof for casting the irradiation light and the wafer 30. This results in a problem in that it is difficult to secure enough space for the shutter 51 to be inserted between the optical system 25 and the shutter 51. Even with an arrangement wherein a shutter 51 is inserted between the projection optical system 25 and the wafer 30, it can be clearly understood that the irradiation light will heat the shutter 51, and in turn, the wafer stage 32 will become heated due to the increase in the shutter's heat. Thus, the adverse effects due to heating will still occur.
In addition, with an arrangement having a mechanical configuration wherein the shutter 51 is driven above the wafer 30, there are problems associated with heating, dust, and the like, occurring from guide members or actuators for driving the shutter 51.